[davidr]

My Nikon D40 will go through stages where it will not register there is a battery in the camera, thus it will not turn on. This problem will, over several weeks correct itself and the camera will work fine for several weeks and then, the problem will arise again. Please help! By the way, the battery is always charged and works fine.

[NT73]

If a battery is always charged up, it may lose the ability to hold a charge. So I am led to believe.
Let the battery run out completely before recharging.
A spare battery will mean you always have power.

Or get a Canon for reliability.

[shuttertalk]

How long have you had the battery, and is it still holding its charge well, compared to when it was new?

With my flashgun, it started playing up until I changed my batteries for new ones.

Also possibly try cleaning the contacts...